First of all, 3GPP (3rd generation partnership project) radio communication system based on WCDMA (wideband code division multiple access) radio access technology is globally developing. In particular, HSPDA (high speed downlink packet access) defined as a first evolution stage of WCDMA provides 3GPP with a radio access technology having high competitiveness in the mid-term future.
E-UMTS provides high competitiveness in the long-term future. The E-UMTS (evolved universal mobile telecommunications system) is the system evolved from a conventional WCDMA UMTS and its basic standardization is ongoing by 3GPP. Generally, E-UMTS can be called LTE (long term evolution) system. For the details of the technical specifications of UMTS and E-UMTS, Release 7 and Release 8 of ‘3rd Generation Partnership Project: Technical Specification Group Radio Access Network’ can be referred to.
E-UMTS mainly consists of a user equipment (UE), a base station and an access gateway (AG) provided to an end terminal of a network (E-UTRAN) connected to an external network. The base station is normally able to simultaneously transmit multi-data stream for a broadcast service, a multicast service and/or a unicast service. In LTE system, OFDM (orthogonal frequency divisional multiplexing) and MIMO (multi-input multi-output) are used to transmit various services in downlink.
The OFDM represents a high speed data downlink access system. The advantage of the OFDM is high spectrum efficiency in enabling all allocated spectra to be used by all base stations. A transmission band in OFDM modulation is divided into a plurality of orthogonal subcarriers in a frequency domain and is also divided into a plurality of symbols in a time domain. Since the OFDM divides the transmission band into a plurality of subcarriers, a bandwidth per subcarrier decreases, while a modulation time per subcarrier increases. As a plurality of the subcarriers are transmitted in parallel, a transmission rate of digital data or symbol of a specific subcarrier becomes lower than that of a single carrier.
The MIMO (multiple input multiple output) system is a communication system that uses a plurality of transmitting and receiving antennas. As the number of the transmitting and receiving antennas is incremented, it is able to linearly increase a channel capacity without increasing a frequency bandwidth additionally. The MIMO technique can be classified into a spatial diversity scheme of raising transport reliability using symbols through various channel paths and a spatial multiplexing scheme of raising a transmission rate in a manner of transmitting separate data streams simultaneously from a plurality of transmitting antennas.
And, the MIMO technologies can be classified into an open-loop MIMO technique and a closed-loop MIMO technique, in accordance with whether a transmitting stage is aware of channel information. In the open-loop MIO technology, a transmitting stage is not aware of channel information. For examples of the open-loop MIMO technique, there are PARC (per antenna rate control), PCBRC (per common basis rate control), BLAST, STTC, random beamforming and the like. On the contrary, in the closed-loop MIMO technique, a transmitting stage is aware of channel information. And, performance of the closed-loop MIMO system depends on how accurately the transmitting stage is aware of the channel information. For examples of the closed-loop MIMO technique, there are PSRC (per stream rate control), TxAA and the like.
The channel information means radio channel information (e.g., attenuation, phase shift, time delay, etc.) between a plurality of transmitting antennas and a plurality of receiving antennas. In the MIMO system, various stream paths attributed to a plurality of transmitting and receiving antenna combinations exist. And, the MIMO system has a fading characteristic that a channel status irregularly changes in time/frequency domain in accordance with time due to multi-path time delay. Therefore, a transmitting stage calculates channel information through channel estimation. In this case, the channel estimation means that channel information is estimated to reconstruct a distorted transmission signal. For instance, channel estimation means that a size and reference phase of a carrier are estimated. In particular, the channel estimation means that a frequency response of a radio interval or channel.
For example of a channel estimating method, there is a method of estimating a reference value based on reference signals (RSs) of several base stations using a 2-dimensional (2D) channel estimator. In this case, the reference signal (hereinafter abbreviated RS) means a symbol having high output despite not having data actually in order to help carrier phase synchronization, base station information acquisition and the like. A transmitting/receiving state is able to perform the channel estimation using such RS. The channel estimation using RS is performed in a manner of estimating a channel through a symbol known in common to transmitting and receiving stages and then reconstructing data using the corresponding estimation value. Besides, the RS can be named a pilot as well.
The MIMO system supports a TDD (time division duplex) system and a frequency division duplex (FDD) system. In the TDD system, since a forward link transmission and a backward link transmission exist in a same frequency domain, estimation can be performed on a forward link channel from a backward link channel by reciprocity principle.